1. Field of the Invention
The subject invention relates to a hypodermic syringe having a sharp needle cannula and a rigid protective shield positioned over the syringe barrel for movement from a position which exposes the needle cannula to a position which surrounds the needle cannula.
2. Description of the Prior Art
Hypodermic syringes typically have a needle shield removably mounted over the needle cannula to avoid accidental needle sticks and to prevent damage to the needle before use. The needle shield can be safely removed and discarded when the hypodermic syringe is about to be used.
Accidental needle sticks occurring after the needle cannula has been used pose a greater health risk, because the used needle cannula may be contaminated. Most health care facilities provide sharps receptacles into which a used hypodermic syringe may be safely deposited. However, the hypodermic syringe is not always used near a sharps receptacle, and the needs of a patient may prevent the health care worker from traveling to the sharps receptacle immediately after using the hypodermic syringe.
The prior art includes hypodermic syringes with a safety shield telescoped over the syringe barrel and movable between a proximal position where the needle cannula is exposed and a distal position where the needle cannula is surrounded. The prior art safety shield is releasably retained in its proximal position until after the needle cannula is used. The safety shield then is moved distally on the syringe barrel to protectively enclose the needle cannula for preventing accidental needle sticks. The shield is either locked in the extended position or releasably retained in this position.
The prior art focuses on structures for holding the safety shield in its distal needle protecting position. Many structures are taught which involve structure on the distal end of the syringe barrel and the proximal end of the safety shield. However, the prior art gives less attention to releasably retaining the safety shield in its proximal needle exposing position. The safety shield must be held to the syringe barrel with enough force to prevent it from becoming dislodged during normal use of the syringe. However, the force must not be so high that the shield cannot be easily moved from the proximal position to the distal needle protecting position. Some designs rely on an interference fit between structure on the inside diameter of the proximal end of the safety shield and the outside diameter of the proximal end of the syringe barrel. However, the frictional engagement places stresses in the safety shield and the barrel, and, over time, the plastic material creeps and the retaining force is reduced. Also, the frictional interference fits are very tolerance dependent. Accordingly, manufacturers may have to design the part initially to require more force to move the safety shield than is ideal, knowing that over time the force will gradually be reduced. Other prior art devices use canitilevered projections from the needle shield to engage a groove or recesses in the proximal end of the syringe barrel. This structure reduces the problems mentioned above, however, it is still subject to creep and tolerance dependency. Also, providing structure on the proximal end of the syringe barrel to accomplish the releasable retention of the safety shield creates additional manufacturing difficulty because both ends of the syringe barrel must have tightly controlled dimensions, with the distal end of the barrel controlling the safety shield in its extended position and the proximal end of the barrel controlling the safety shield in its retracted proximal position. Also, structure on the proximal end of the syringe barrel can greatly increase manufacturing costs due to the injection molding process wherein it becomes difficult to remove the syringe from the mold because of this structure. The mold can, at much greater cost, be made to split along the longitudinal axis of the barrel or have cam actuated elements to allow easy removal of the molded barrel. Without the more complex mold, the structure on the barrel may have to be so small to enable the barrel to be removed from the mold that it may not be as effective as desired.
Although the prior art provides many syringes having safety shields and many structures to hold the safety shield in the extended needle protecting position and to releasably retain the safety shield in its proximal needle exposing position, there is still a need for simple, straight-forward easy-to-manufacture syringe having a safety shield wherein both the extended and the retracted position of the safety shield are controlled by structure on the distal end of the syringe barrel using a structure that does not subject the safety shield and the syringe barrel to great stresses while the syringe is in storage with the safety shield in its proximal position with respect to the syringe barrel.